Recently, it has been increasingly demanded to restrain the consumption of fossil fuels in order to control global warming and protect the environment, which has affected various manufacturing industries. For example, automobiles, which are an indispensable part of transportation means in daily life and activities, are not exception. There is a demand to improve fuel economy by, for example, reducing vehicle body weight. It is not allowed, however, to simply reduce the vehicle body weight by neglecting product qualities. It is necessary to secure appropriate safety.
Many of the structural parts of an automobile are made of ferrous material, in particular a steel sheet. For reducing the vehicle body weight, it is important to reduce the weight of the steel sheet. Instead of simply reducing the weight of the steel sheet, which is not allowed as mentioned above, the weight reduction must be accompanied with obtaining the mechanical strength of the steel sheet. Such demand becomes higher not only in the car manufacturing industry but also in various other manufacturing industries. Research and development efforts have been directed to a steel sheet that can have the same or a larger mechanical strength as compared to conventional one even when the sheet is made thinner.
In general, a material having a high mechanical strength tends to become lower in formability and shape fixability in shape formation work such as bending. It is difficult to carry out the process for forming such material into a complicated shape. One of the solutions to the formability problem is what is called “a hot pressing method (also referred to as hot stamping, hot pressing, die quenching, or press hardening)”. In the hot pressing method, a material to be formed is heated temporarily to a high temperature (in an austenite region) and the steel sheet soften by the heating is formed by pressing. The steel sheet is then cooled. By using the hot pressing method, the material is once soften by heating to a high temperature so that the material is easy to be pressed. The mechanical strength of the material becomes larger due to a quenching effect during cooling after the shaping. Accordingly, the hot pressing can provide a product having both a good shape fixability and a high mechanical strength.
When the hot pressing method is applied to a steel sheet, however, iron and other substances on the surface are oxidized to generate scales (oxides) due to heating to a high temperature of, for example, 800° C. or more. Accordingly, a descaling process is necessary after hot pressing to remove the scales, which deteriorates productivity. For the members and the like that require corrosion resistance, it is necessary to carry out anti-corrosion treatment and metal cover installation on the surfaces of the members after the shaping process. A surface cleaning process and a surface treatment process are also necessary, which further deteriorates productivity.
As an example of restraining such deterioration in productivity, a covering layer can be installed on a steel sheet. In general, various materials including organic and inorganic materials are used for the covering layer on a steel sheet. Among them, galvanized steel sheets that have a sacrificial protection effect on steel sheets are widely used for steel sheets for automobiles and other products because the galvanized steel sheets provide a good anti-corrosion effect and suitability to steel sheet production technology. However, this may cause to considerable deterioration in the surface properties because heating temperatures used in the hot pressing (700 to 1000° C.) are higher than the temperatures at which the organic materials decompose or the zinc boils so that the plating layer evaporates at a time of heating by hot press.
For this reason, it is desirable to use, for example, what is called an Al-plated steel sheet for the hot pressing that heats the steel sheet to high temperatures. The Al-plated steel sheet is a steel sheet having an Al-based metal cover that has the boiling point higher than that of an organic material cover or Zn-based metal cover.
The Al-based metal cover can prevent scales from depositing on the surface of the steel sheet, which leads to omitting a process such as the descaling process and improving productivity. The Al-based metal cover also has an anti-corrosion effect so that the corrosion resistance of the steel sheet after coated with paint is improved. Patent Literature 1 listed below discloses a method for using an Al-plated steel sheet in hot pressing, the Al-plated steel sheet being obtained by covering a steel sheet having predetermined steel components with Al-based metal, as explained above.
However, in the case that the Al-based metal cover is applied like Patent Literature 1, the Al cover is melted and transformed into an Al—Fe compound due to the dispersion of Fe from the steel sheet, depending on preheating conditions before a pressing step in the hot pressing method. The Al—Fe compound grows until the Al—Fe compound reaches to the surface of the steel sheet. The compound layer is hereinafter referred to as the Al—Fe alloy layer. The Al—Fe alloy layer is so hard. That is because the Al—Fe alloy layer is intrinsically not smooth on the surface and is inferior in lubricity, comparatively. In addition, since the Al—Fe alloy layer tends to break, develop cracks in a plating layer, and come off in a powder form. Moreover, flaked materials from the Al—Fe alloy layer and coming-off materials by strong abrasion on the Al—Fe surface attach on the dies. The Al—Fe compound then adheres to and deposits on the dies, which leads to deterioration in the quality of pressed products. To prevent this, it is necessary to remove Al—Fe alloy powder adhered to the dies during maintenance, which is one of the causes for lowering productivity and increasing the cost.
Furthermore, the Al—Fe alloy layer is less reactive in phosphate treatment so that a chemical conversion coating (a phosphate coating), which is a treatment before electrodeposition painting, is difficult to generated. Although the chemical conversion coating is not formed, the Al—Fe alloy layer itself has a good coating adhesion ability with paint so that corrosion resistance after coated with paint becomes better if Al plating amount is large enough. An increase in the amount, however, tends to worsen the aforementioned adhesion to the dies.
On the other hand, Patent Literature 2 listed below discloses a technique in which a wurtzite-type compound is applied to the surface of an Al-plated steel sheet. According to the Patent Literature 2 listed below, such a process improves in lubricity in hot state and in chemical conversion treatability. This technique is effective for improving lubricity and also corrosion resistance after coated with paint.
In addition, Patent Literature 3 listed below discloses a technique for controlling the average section length of the crystal grains that are in an intermetallic compound phase and contain Al at an amount of 40% or more and 65% or less among the crystal grains of Al—Fe that is a main ingredient of the intermetallic compound phase formed on the surface of the steel sheet, and also for controlling the thickness of the intermetallic compound phase. The technique also includes forming of a lubricating coating containing ZnO on the surface of the Al plating layer. In Patent Literature 3 listed below, the corrosion resistance after coated with paint and the formability during hot stamping can be improved by using such techniques.